These are all from here: https://www.kickstarter.com/projects/1002143342/undertale/posts/1087879
That aside, Toby is likely referring to Hotland.
“Area 4 Boss was finished. It was the least intuitive to make, so it took a long time. Eventually, you will play it and wonder why it took so long.”
Took so long?
YOUR TAKING TOO LONG?
“After you reach a certain point in the game, the monsters become more difficult. Then the battle theme changes. If you hear this song, things will be hard for you from now on.”
(Toby seems to be referring to Stronger Monsters, though the audio won't play.)
The battle theme changes?
Things will be hard for you from now on?
From Now On (Battle Theme 2)?
“Oh... it's from last year... hmmm... well, I'm sorry. I can't make a second one. A game with two Christmas trees... that's just against the rules.”
Toby already broke this rule with Chapters 3 and 4.
Chapter 3 Cold Place has a Christmas tree that Toriel sits on, while Chapter 4 Holiday Manor has a Christmas tree in the basement, kitchen, as well as bathroom .
Potential proof that the Weird Route is Prophecy following? (On Weird Route, you never see the basement.)